1. Description of Related Art
In the medical arts, to relieve undesirable accumulation of fluids, it is frequently necessary to provide a means for draining a fluid from one part of the human body to another in a controlled manner. This is required, for example, in the treatment of hydrocephalus, an ailment usually afflicting infants or children in which cerebrospinal fluid accumulates within the skull and exerts pressure and skull deforming forces. The aim of treatment in hydrocephalus is to return the excess cerebrospinal fluid accumulating in the brain ventricles back to the venous circulation. This is done either directly by shunting it to the venous circulation, or indirectly by shunting it to a cavity, like the peritoneal cavity, from where it is absorbed into the venous circulation. This is typically done utilizing a drainage or shunt system including a catheter inserted into the brain ventricle through the skull which is connected to a tube that conducts the cerebrospinal fluid away from the brain to be introduced either into the peritoneal cavity, i.e. a ventriculo-peritoneal shunt system, or into the venous circulation by extending the distal catheter through the patient's internal jugular vein to the atrium portion of the heart, i.e. a ventriculo-atrial shunt system. After implantation of such shunt systems, two major problems are encountered: siphonage and thrombo-embolic complications.
Siphonage inevitably occurs whenever the patient assumes an upright posture regardless of the shunt system used. Siphonage develops whenever the ventricular catheter inlet, which is inside the skull, is elevated with respect to the distal catheter outlet, which is in the peritoneal cavity or the right atrium, i.e. when the patient sits, stands or is held erect. This siphonage effect occurs as a result of the hydrostatic pressure of the cerebrospinal fluid column in the vertical segment of the shunting catheter and causes cerebrospinal fluid over-drainage. Over-drainage of the cerebrospinal fluid causes excessive reduction of the cerebrospinal fluid pressure within the brain ventricles and predispose for the development of several complications, e.g., subdural hematomas and hygromas, overriding of the cranial bones and premature union of the skull bones in infants, excessive reduction of the ventricular size leading to the slit ventricle syndrome and shunt obstruction due to suction of choroid plexus or brain debris or due to impingement of the ventricular walls on the inlet holes of the ventricular catheter . . . etc. Furthermore, when a patient assumes the recumbent position after a period of over drainage in the upright position, cerebrospinal fluid flow to draining site stops completely until the intra-ventricular pressure builds up again; in effect, flow of cerebrospinal fluid to the drainage sites after implantation of current shunts is intermittent according to changes in posture or intra-thoracic pressure. Intermittent flow of the fluid predisposes to shunt obstruction and infection as a result of stagnation of the cerebrospinal fluid or reflux of blood into the shunting catheter. To sum up, siphonage is responsible, either directly or indirectly, for most of the problems and complications encountered after implantation of current shunts.
The thrombo-embolic complications, which are particularly peculiar to ventriculo-venous shunts, occur as a result of clotting of blood at the venous end of the connection. These are serious, fatal complications and because they are difficult to treat, ventriculo-atrial shunts are rarely implanted nowadays. The ventriculo-peritoneal shunt is the one most commonly used for the treatment of hydrocephalus.
Fluid flow control devices are incorporated into the shunt systems used for treatment of hydrocephalus, with the aim of maintaining a normal intra-ventricular pressure, prevention of siphonage in the upright body position and prevention of reflux of fluids into the shunt system. These devices are combinations of check valves, differential pressure valves and anti-siphonage devices. Although incorporation of these devices into shunt systems have improved the results, yet they are still far from satisfactory. Complications still occur frequently, the incidence of shunt revision is still high and the search to find solutions for the problems encountered after implantation of current shunt systems continues.
2. Evolution of the Concept of Treating Hydrocephalus by Implanting Retrograde Ventriculo-Sinus Shunts
The concept of treating hydrocephalus by implanting retrograde ventriculo-sinus shunts, i.e. by establishing a water tight connection to drain excess cerebrospinal fluid accumulating in hydrocephalic ventricles and to deliver it into a dural sinus against the direction of blood flow in the sinus, evolved from two theoretical conclusions. The first conclusion addressed the problem of siphonage and the second conclusion addressed the problems of regulation of cerebrospinal fluid flow to the venous circulation, normalization of the intraventricular pressure, prevention of reflux of blood and prevention of blood clotting at the venous end of a ventriculo-venous connection.
First Conclusion
The first conclusion was based on the fact that veins are collapsible tubes. The observation that veins may collapse in vivo is an old one and has traditionally been attributed to the fact that their walls are too thin to withstand even minor changes in their transmural pressure (N.B. Transmural pressure is the pressure inside the vein minus the ambient pressure). Since the pressure in the big veins in the body oscillates around a value close to atmospheric pressure, conditions under which transmural pressure drops below the zero level may prevail; investigators suggested that collapse of veins may play a role in regulation of the venous flow [A. L. Noordergraaf, Circulatory System Dynamics, Ch. 5, (1978)]. The internal jugular vein is the only segment in the venous channels that convey the venous blood from the brain to the heart which is exposed to the atmospheric pressure. In the recumbent position the vein lies in a horizontal plane, it's transmural pressure is just positive and it's contour is oval or even flat. During change of posture form recumbent to erect, the position of the vein changes from horizontal to vertical allowing gravity to exert it's effect decreasing the vein's transmural pressure which becomes negative and the vein collapses.
Dr. El-Shafei conducted experiments to study the role of collapse of the internal jugular vein in regulating the venous back flow from the brain during changes in posture [Ismail L. El Shafei, et al., “Ventriculo-Jugular Shunt Against the Direction of Blood Flow, I. Role of the Internal Jugular Vein as an Antisiphonage Device”, Childs Nervous System, 3:282-284 (1987)]. He found that the internal jugular vein collapsed gradually, in a specific pattern, during change of posture from recumbent to erect. Collapse of the vein started at its upper end and proceeded downward with the increase in the angle of elevation of the cranium above the heart. The degree and extent of collapse of the vein varied according to the degree of elevation of the cranium; the more the cranium was elevated the more the vein collapsed. This phenomenon is similar to what happens to the veins on the dorsum of the hand when it is elevated. Collapse of the vein decreased it's capacity and increased the resistance to blood flow in it, counteracting the gain in pressure head due to elevation of the cranium. The increase in resistance to blood flow due to collapse of the vein was exactly equal to the gain in pressure head due to elevation of the cranium, so that the rate of venous back flow from the brain as well as all the intracranial pressures, i.e. the dural sinus pressure, the cerebral venous pressure and the cerebrospinal fluid pressure, remained constant and were not affected by gravity during the change of posture from recumbent to erect. In other words, collapse of the internal jugular vein on assuming the erect posture acted as a natural, precise and self regulating siphon control mechanism. Accordingly, Dr. El-Shafei concluded that: “any cerebrospinal fluid shunt for treatment of hydrocephalus should deliver the cerebrospinal fluid into the upper end of the internal jugular vein or into one of the dural sinuses in order to utilize the natural phenomenon of collapse of the internal jugular vein on assuming the upright body position to prevent siphonage”.
Second Conclusion
The second conclusion was based on general hydraulic principles. In any ventriculo-venous shunt, the intra-venous portion of the shunting catheter constitutes an obstacle in the way of the running blood stream. According to hydraulic principles, placement of an obstacle in the way of a running stream induces changes in the pattern of flow and in the distribution of pressures inside the stream [W. L. McCabe, J. C. Smith, Unit Operations of Chemical Engineering, 2nd Edition, Ch. 3 “Fluid-flow Phenomena”, (1967) pp 47-67]. These changes are well known to hydraulic and aviation engineers. An impact zone will be created on the upstream side of the obstacle and a wake zone will be created on its downstream side (FIG. 1A). In the impact zone, the running fluid hits the obstacle, deflects and moves on, i.e. the fluid never stagnates in the impact zone, and is continuously renewed, while in the wake zone, some of the fluid separates from the streamlines and stagnates. Also, in the impact zone, the static pressure of the running fluid increases by an amount known as the impact pressure, while in the wake zone it decreases by an amount known as the wake effect. (N.B. The magnitude of the impact pressure or wake effect is directly proportional to the square of the velocity of fluid flow and equals V2/2 g, where V is the velocity of flow and g is the gravitational acceleration which is a constant equal to 981 cm/s2).
According to this hydraulic principle, if the shunting catheter of a ventriculo-venous shunt is introduced into the venous channel in the direction of blood flow (FIG. 1B), two wake zones will be created in the venous channel; one in front of the catheter's end (WZ1 in FIG. 1B) and another at the axilla between the catheter and the wall of the venous channel (WZ2 in FIG. 1B). At these sites, blood will stagnate and it's clotting will be encouraged (N.B. Blood clots when it stagnates, but not when it is continuously renewed). Also, in these wake zones, the static venous pressure will decrease by an amount equal to the wake effect of the blood stream. Accordingly, the intra-ventricular pressure will stabilize after shunt implantation, when it becomes equal to the wake zone pressure in front of the venous end of the shunting catheter, i.e. the stabilized intra-ventricular pressure will be less than the pressure in the draining venous channel by an amount equal to the wake effect of the blood stream; i.e. the normal relationship of the intraventricular pressure being more than the pressure in the draining venous channel will be reversed.
On the other hand, if the shunting catheter of a ventriculo-venous shunt is introduced into the venous channel against the direction of blood flow (FIG. 1C), two impact zones will be created in the venous channel, one in front of the catheter's venous end (IZ1 in FIG. 1C) and another at the axilla between the catheter and the wall of the venous channel (IZ2 in FIG. 1C). In the impact zones blood will never stagnate; it will be continuously renewed and its clotting will be discouraged. Also, the intraventricular pressure will stabilize after shunt implantation when it becomes equal to the impact zone pressure in front of the catheter's venous end, i.e., the stabilized intraventricular pressure after shunt implantation will be maintained more than the pressure in the draining venous channel by an amount equal to the impact pressure of the blood stream, preserving the relationship that normally exists between the two pressures. Accordingly, it was concluded that: “in order to discourage blood clotting at the venous end of a ventriculo-venous shunt and to normalize the intraventricular pressure and maintain it more than the pressure in the draining venous channel regardless of changes in posture or intra-thoracic pressure, the venous end of the shunting catheter should be introduced into the venous channel against the direction of blood flow and not in the direction of blood flow”.
To sum up, on theoretical basis, Dr. El Shafei concluded that “a physiological way for treating hydrocephalus is to establish a closed watertight connection that will drain the excess cerebrospinal fluid from the cerebral ventricles and deliver it into the upper end of the internal jugular vein or into a dural sinus against the direction of blood flow, i.e. to implant a retrograde ventriculo-jugular or a retrograde ventriculo-sinus shunt”. Such a treatment utilizes the collapse of the internal jugular vein in the erect posture to prevent siphonage, utilizes the impact pressure of the blood stream to maintain the normal relationship of the intra-ventricular pressure being more than the pressure in the draining venous channel regardless of changes in posture or intra-thoracic pressure and creates impact zones in the draining venous channel which discourage stagnation and clotting of blood at the venous end of the connection. This way of cerebrospinal fluid shunting mimics natural drainage of cerebrospinal fluid and eliminates almost all the predisposing factors for most of the problems and complications encountered after current shunting procedures. These theoretical conclusions needed confirmation.
Dr. El-Shafei et al. conducted experiments to study the dynamics of cerebrospinal fluid flow after implantation of retrograde ventriculo-jugular and retrograde ventriculo-sinus shunts respectively [Ismail L. El-Shafei and Mahmound A. El-Rifaii, “Ventriculo-jugular shunt against the direction of blood flow. II Theoretical and experimental basis for shunting the cerebrospinal fluid against the direction of blood flow”, Child's Nervous System, (1987) 3:285-291 and Azza Mohamed Abdel Hamid El Shaarawy, “Study of Alternative Methods for Cerebrospinal Fluid Shunting to the Venous Circulation, Experimental Modeling”, Thesis submitted to the Faculty of Engineering at Cairo University in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemical Engineering, March 1997]. The results showed that after implantation of either of these shunts, provided that the ventriculo-venous connection is a closed system that does not allow leakage of cerebrospinal fluid, the intraventricular pressure will be regulated by its natural regulator, which is the pressure in the draining venous channel. The intra-ventricular pressure will be maintained within normal values at a level more than the pressure in the draining venous channel regardless of changes in posture or intra-thoracic pressure; therefore blood cannot regurgitate into a valveless shunting catheter as a result of these changes, flow of cerebrospinal fluid to the venous circulation will be continuous at a rate equal to and dependent upon the rate of its formation and siphonage will not develop when the upright body position is assumed. The results confirmed the theoretical conclusions and indicated that a valveless shunting catheter can be used to establish the retrograde ventriculo-venous connection. Reflux of blood into a valveless shunting catheter after implantation of a retrograde ventriculo-venous shunt can only occur if there is cerebrospinal fluid leakage, because loss of the cerebrospinal fluid will decrease the intra-ventricular pressure and make it less than the pressure in the draining venous channel. Therefore, injury to the dura mater should be avoided during shunt implantation and the dural hole made for shunt implantation should be hermetically sealed to prevent leakage from around the catheter. Also the shunt should be closed before attempting intradural surgical procedures and if a cerebrospinal fluid fistula develops. This is important because the dynamics of cerebrospinal fluid flow after implantation of retrograde ventriculo-venous shunts depend entirely on it's being a closed system allowing no leakage of cerebrospinal fluid.
During the 8th and 9th decades of the 20th century, Dr. El-Shafei implanted retrograde ventriculo-jugular shunts to treat hydrocephalus [Ismail L. El-Shafei, “Ventriculo-jugular shunt against the direction of bloodflow, III Operative technique and results”, Child's Nervous System, (1987) 3:342-349 and Ismail El-Shafei and Mohamed Ahmed Hafez, “Ventriculo-jugular shunt against the direction of blood flow, IV Technical modifications and policy for treatment”, Child's Nervous System (1991) 7:197-204]. The results were satisfactory in patients above the age of 5 years. There were no problems related to improper cerebrospinal fluid drainage or blood clotting; confirming the validity of the theoretical conclusions and the experimental results. However, implantation of a retrograde ventriculo-jugular shunt was very difficult and quite often impossible in infants and young children due to the thin caliber of their neck veins. Also, when the shunt could be implanted in young patients, only a short segment of the shunting catheter could be introduced up into the thin internal jugular vein. Shortness of the intravenous catheter segment together with movements of the head and the rapid rate of longitudinal body growth in infants predisposed to slipping of the catheter out from the vein and cerebrospinal fluid leakage in the neck few months after shunt implantation. It became obvious that infants and young children are not suitable candidates for implantation of retrograde ventriculo-jugular shunts, and they were only implanted in patients above the age of 5 years.
During the last decade of the 20th century, Dr. El Shafei, et al, implanted retrograde ventriculo-sinus shunts using valveless shunting catheters regardless of the patient's age [El Shafei I L, El Shafei H I, “The retrograde ventriculo-sinus shunt. Concept and technique for treatment of hydrocephalus by shunting the cerebrospinal fluid to the superior sagittal sinus against the direction of blood flow. Preliminary report. Childs' Nerv. Syst. 2001; 17:456-465, Ismail L El Shafei, Hassan I E L Shafei, “The Retrograde Ventriculo-Sinus Shunt (El Shafei RVS Shunt). Rationale, Evolution, surgical technique and long term results.” Pediatr Neurosurg 2005; 41:305-317]. The results were considered satisfactory, there were no problems related to improper cerebrospinal fluid drainage, siphonage, reflux of blood or blood clotting at the venous end of the connection. However, difficulties and problems were encountered during and after shunt implantation mainly due to cerebrospinal fluid leakage from around the shunting catheter because of imperfect sealing of the dural hole made for shunt implantation and difficulties in introducing the venous end of the connection into the sinus.
Conclusion: Since none of the available shunt systems possess features for hermetic sealing of the dural hole around the shunting catheter or for delivering the drained cerebrospinal fluid against the direction of blood flow in the sinus it became obvious that a specially designed shunt system is needed for easy and proper implantation of retrograde ventriculo-sinus shunts.